In normal operation, magnetic memory cells such as spin-torque magnetic memory cells store data in magnetic tunnel junctions based on the magnetic orientation of a free portion relative to a fixed portion, the free and fixed portions being separated by a tunnel barrier. In-plane and perpendicular embodiments are known in the art and refer to the orientations of the magnetic easy axes of free and fixed layers relative to the memory device film plane. In such memory cells, the voltage drop across a magnetic tunnel junction (MTJ) in each memory cell can be varied based on the relative magnetic states of the ferromagnetic layers within the MTJ device. Because the resistance through the memory cell changes based on the magnetic orientation of the free portion, information can be stored by setting the orientation of the free portion. The information is later retrieved by sensing the orientation of the free portion which is indicated by the resistance across the memory cell.
Writing to spin-torque magnetic memory cells is accomplished by passing a write current through the MTJ device where the angular momentum carried by the spin-polarized tunneling current can change the magnetic state of the free portion. Depending on the direction of the current through the MTJ device, the resulting magnetization of the free portion will either be in a first state or a second state relative to the fixed portion. In some spin torque devices, the first and second states correspond to the free portion magnetization being parallel to the fixed portion magnetization and anti-parallel to the fixed portion magnetization, respectively. If the parallel orientation represents a logic “0”, the antiparallel orientation represents a logic “1”, or vice versa. Thus, the direction of write current flow through the MTJ device determines whether the memory cell is written to a first state or a second state. In such memories, the magnitude of the write current is typically greater than the magnitude of a read current used to sense the information stored in the memory cells.
In some applications, it is useful to store a set of data in memory cells during manufacturing. For example, it may be desirable to store security codes or boot-up software on an integrated circuit during manufacturing so it can later be used when the integrated circuit is included in a system. However, because the data retention ability of magnetic memory cells is adversely affected by high temperatures, later manufacturing steps that include high-temperature operations (e.g. packaging and soldering) that can cause normal magnetic memory cells to lose any data stored in those cells prior to such high-temperature operations. Similarly, exposure to strong magnetic fields after being programmed to a particular state during manufacturing can also result in data loss. As such, there is a need for techniques and circuitry to support the storage and retrieval of data in spin-torque magnetic memory cells in a manner that is less susceptible to exposure to high temperatures or external magnetic fields.